The objective is to study the natural history of pneumonia virus of mice (PVM), an endemic/epidemic respiratory pathogen of laboratory rodents including mice, rats, hamsters, guinea pigs, and probably others. Little is known about the pathology, patent period, epidemiology, and target cell population of this virus. PVM is currently classified as a Pneumovirus of the family Paramyxoviridae. It shares this classification with respiratory syncitial virus (RSV) of man, and bovine respiratory syncitial virus (BRSV) of cattle. Minor differences are seen in the electrophoretic mobility patterns of the polypeptides of various isolants of RSV. These differences are not distinguishable with human convalescent serum but are distinguishable with individually prepared antisera. Similar statements may be made about BRSV, although the information is not as strong. Furthermore, RSV and BRSV have similar polypeptide profiles indicating close relatedness. PVM on the other hand has several major differences. We have observed, and will report for the first time, fatal wasting disease in athymic nu/nu mice naturally infected with PVM. Immunofluorescence studies of lung material from index cases indicate that the target cell in late stage natural infection in the nu/nu mouse is located in the pulmonary parenchyma rather than the lung airways. Because of the broad spectrum of rodent hosts, PVM has the potential to influence many experimental results, notably, those studies involving the cell dynamics of pulmonary parenchyma.